1. Technical Field
The present invention generally relates to a polishing tool for polishing a workpiece and, more particularly, to a conditioner for a polishing tool for polishing a workpiece such as a semiconductor wafer.
2. Description of the Related Art
Machines for preparing and fabricating semiconductor wafers are known in the art. Wafer preparation includes slicing semiconductor crystals into thin sheets, and polishing the sliced wafers to free them of surface irregularities, that is, to achieve a planar surface. In general, the polishing is accomplished in at least two steps. The first step is a rough polishing or abrasion. This step may be performed by an abrasive slurry lapping process in which a wafer mounted on a rotating carrier is brought into contact with a rotating polishing pad upon which is sprayed a slurry of insoluble abrasive particles suspended in a liquid. Material is removed from the wafer by the mechanical buffing action of the slurry. The second step is fine polishing. The fine polishing step is performed in a similar manner to the abrasion step, however, a slurry containing less abrasive particles is used. Alternatively, a polishing pad made of a less abrasive material may be used. The fine polishing step often includes a chemical mechanical polishing ("CMP") process. CMP is a combination of mechanical and chemical abrasion, and may be performed with an acidic or basic slurry. Material is removed from the wafer due to both the mechanical buffing and the action of the acid or base. Such polishing is also important during the manufacturing of semiconductor devices in order to planarize various thin film layers formed on the surface of a semiconductor wafer. The thin film may, for example, be an interlayer insulating film formed between two metal layers, a metal layer, an organic layer, or a layer of semiconductor material. Polishing apparatus are disclosed, for example, in U.S. Pat. Nos. 5,245,796 and 5,216,843.
One important factor to achieving and maintaining a high and stable polishing rate is pad conditioning. Conditioning is generally performed after each wafer is polished in order to remove debris and grit and to make the pad surface rough. By this procedure, the pad can absorb or hold enough fresh slurry to achieve a high and stable polishing rate. Conditioning also contributes to the uniformity of polishing.
A wafer polishing apparatus 10 is generally shown in FIG. 1. Wafer polishing apparatus 10 includes a pad conditioning assembly 12 and a wafer carrier assembly 14. Pad conditioning assembly 12 includes a shaft 15 which is connected to a conditioning element carrier 16. Shaft 15 includes a joint 18 such as a ball and socket joint for maintaining planar contact between the bottom surface 20 of diamond pellet carrier 16 and a pad 22. Shaft 15 may be rotated by a motor (not shown) to impart rotational motion to carrier 16. Wafer carrier assembly 14 includes a carrier 42 for applying a downward pressure against the backside of a wafer 44. The backside of wafer 44 is held in contact with the bottom of carrier 42 by a vacuum or by wet surface tension. An insert pad 46 may be provided between the backside of wafer 44 and carrier 42. Carrier 42 includes downwardly extending sidewall portions to prevent wafer 44 from slipping laterally from under carrier 42 during processing. The downward pressure is applied by means of a shaft 48 connected to carrier 42. Shaft 48 includes a joint 50 to maintain planar contact between carrier 42 and polishing pad 22. Shaft 48 may be rotated by a motor (not shown) to rotate wafer 44 and enhance the polishing process. As can be seen with reference to FIGS. 1 and 2, conditioning elements 24 project outwardly from bottom surface 20 of conditioning element carrier 16. Conditioning elements 24 may, for example, be diamond conditioning elements. As shown in views of FIGS. 3A and 3B, in operation, conditioning elements 24 of pad conditioner assembly 12 are embedded into pad 22 and pad conditioner assembly 12 is rotated in a counterclockwise direction at a rate of 60 rotations per minute (RPM), for example. Polishing pad 22 is fixedly attached to an upper surface of a rotatable table 25. The pad may, for example, be an IC-1000/SUBA-N double layered pad. In operation, table 25 is rotated in a counterclockwise direction at a rate of 100 RPM, for example. The rotational motion of table 25 may be provided by a motor (not shown).
As indicated in FIG. 3B, pad conditioner assembly 10 is subject to unstable motion during operation. For example, as shown in FIG. 4A, during the conditioning operation, a high friction point 30 can exist between conditioning elements 24 and polishing pad 22. If the conditioning assembly 12 is moving to the left as indicated by the arrow in FIG. 4A, conditioning element carrier 16 can "skip" as shown in FIG. 4B. This skipping generates an unstable motion which creates a topography 32 on the surface of polishing pad 22 as shown in FIG. 3C and results in non-uniformity of the polishing of the wafer. This effect is illustrated in FIGS. 5A and 5B. FIG. 5A illustrates the polishing rate in .ANG.ngstroms per minute as a function of position across the wafer. It can be seen with reference to FIG. 5A that the polishing rate is fairly uniform and ranges from about 900 to about 1000 .ANG.ngstroms per minute after conditioning has been performed three times. However, as can be seen with reference to FIG. 5B, after conditioning has been performed thirty times, the polishing rate varies from less than 800 to more than 1000 .ANG.ngstroms per minute across the wafer surface. In particular, the polishing rate of at the center of the wafer becomes very slow because the down-force or pressure at the center of the wafer is low due to the topography 32 on polishing pad 22.